A major portion of the worldwide petrochemical industry is concerned with the production of light olefin materials and their subsequent use in the production of numerous important chemical products via polymerization, oligomerization, alkylation and the like well-known chemical reactions. Light olefins include ethylene, propylene and mixtures thereof. These light olefins are essential building blocks for the modern petrochemical and chemical industries. The major source for this materials in present day refining is the steam cracking of petroleum feeds. For various reasons including geographical, economic, political and diminished supply considerations, the art has long sought a source other than petroleum for the massive quantities of raw materials that are needed to supply the demand for these light olefin materials. In other words, the holy grail of the R & D personnel assigned to work in this area is to find a way to effectively and selectively use alternative feedstocks for this light olefin production application thereby lessening dependence of the petrochemical industry on petroleum feedstocks. A great deal of the prior art's attention has been focused on the possibility of using hydrocarbon oxygenates and more specifically methanol as a prime source of the necessary alternative feedstock. Oxygenates are particularly attractive because they can be produced from such widely available materials as coal, natural gas, recycled plastics, various carbon waste streams from industry and various products and by-products from the agricultural industry. The art of making methanol from these types of raw materials is well established and typically involves the use of one or more of the following procedures: (1) manufacture of synthesis gas by any of the known techniques typically using a nickel or cobalt catalyst followed by the well-known methanol synthesis step using relatively high pressure with a copper-based catalyst; (2) selective fermentation of various organic agricultural products and by-products in order to produce oxygenates; or (3) various combinations of these techniques.
Given the established and well-known technologies for producing oxygenates from alternative non-petroleum raw materials, the art has focused on different procedures for catalytically converting oxygenates such as methanol into the desired light olefin products. These light olefin products that are produced from non-petroleum based raw materials must of course be available in quantities and purities such that they are interchangeable in downstream processing with the materials that are presently produced using petroleum sources. Although many oxygenates have been discussed in the prior art, the principal focus of the two major routes to produce these desired light olefins has been on methanol conversion technology primarily because of the availability of commercially proven methanol synthesis technology. A review of the prior art has revealed essentially two major techniques that are discussed for conversion of methanol to light olefins. The first of these MTO processes is based on early German and American work with a catalytically conversion zone containing a zeolitic type of catalyst system. Representative of the early German work is U.S. Pat. No. 4,387,263 which was filed in May of 1982 in the U.S. without a claim for German priority. This '263 patent reports on a series of experiments with methanol conversion techniques using a ZSM-5-type of catalyst system wherein the problem of DME recycle is a major focus of the technology disclosed. Although good yields of ethylene and propylene were reported in this '263 patent, they unfortunately were accompanied by substantial formation of higher aliphatic and aromatic hydrocarbons which the patentees speculated might be useful as an engine fuel and specifically as a gasoline-type of material. In order to limit the amount of this heavier material that is produced, the patentees of the '263 patent propose to limit conversion to less than 80% of the methanol charged to the MTO conversion step. This operation at lower conversion levels necessitated a critical assessment of means for recovering and recycling not only unreacted methanol but also substantial amounts of a DME intermediate product. The focus then of the '263 patent invention was therefore on a DME and methanol scrubbing step utilizing a water solvent in order to efficiently and effectively recapture the light olefin value of the unreacted methanol and of the intermediate reactant DME.
This early MTO work with a zeolitic catalyst system was then followed up by the Mobil Oil Company who also investigated the use of a zeolitic catalyst system like ZSM-5 for purposes of making light olefins. U.S. Pat. No. 4,587,373 is representative of Mobil's early work and it acknowledged and distinguished the German contribution to this zeolitic catalyst based MTO route to light olefins. The inventor of the '373 patent made two significant contributions to this zeolitic MTO route the first of which involved recognition that a commercial plant would have to operate at pressure substantially above the preferred range that the German workers in this field had suggested in order to make the commercial equipment of reasonable size when commercial mass flow rates are desired. The '373 patent recognized that as you move to higher pressure for the zeolitic MTO route in order to control the size of the equipment needed for commercial plant there is a substantial additional loss of DME that was not considered in the German work. This additional loss is caused by dissolution of substantial quantities of DME in the heavy hydrocarbon oil by-product recovered from the liquid hydrocarbon stream withdrawn from the primary separator. The other significant contribution of the '373 patent is manifest from inspection of the flow scheme presented in FIG. 2 which prominently features a portion of the methanol feed being diverted to the DME absorption zone in order to take advantage of the fact that there exist a high affinity between methanol and DME thereby downsizing the size of the scrubbing zone required relative to the scrubbing zone utilizing plain water that was suggested by the earlier German work.
Primarily because of an inability of this zeolitic MTO route to control the amounts of undesired C4+ hydrocarbon products produced by the ZSM-5 type of catalyst system, the art soon developed a second MTO conversion technology based on the use of a non-zeolitic molecular sieve catalytic material. This branch of the MTO art is perhaps best illustrated by reference to UOP's extensive work in this area as reported in numerous patents of which U.S. Pat. No. 5,095,163, U.S. Pat. No. 5,126,308 and U.S. Pat. No. 5,191,141 are representative. This second approach to MTO conversion technology was primarily based on using a catalyst system comprising a silicoaluminophosphate molecular sieve (SAPO) with a strong preference for a SAPO species that is known as SAPO-34. This SAPO-34 material was found to have a very high selectivity for light olefins with a methanol feedstock and consequently very low selectivity's for the undesired corresponding light paraffins and the heavier materials. This SAPO catalyzed MTO approach is known to have at least the following advantages relative to the zeolitic catalyst route to light olefins: (1) greater yields of light olefins at equal quantities of methanol converted; (2) capability of direct recovery of polymer grade ethylene and propylene without the necessity of the use of extraordinary physical separation steps to separate ethylene and propylene from their corresponding paraffin analogs; (3) sharply limited production of by-products such as stabilized gasoline; (4) flexibility to adjust the product ethylene-to-propylene weight ratios over the range of 1.5:1 to 0.75:1 by minimal adjustment of the MTO conversion conditions; and (5) significantly less coke make in the MTO conversion zone relative to that is experienced with the zeolitic catalyst system.
Despite the promising developments associated with the SAPO-catalyzed route to light olefins, there are still substantial hurdles to overcome before an economically attractive SAPO-catalyzed MTO process can be fully realized. One very substantial economic problem is associated with the start-up of such an MTO process. One unusual feature of an MTO process is the very large amount of steam that is present in the effluent from the MTO conversion zone. The stoichiometry of the MTO reaction is such that for every mole of methanol charged to the MTO conversion zone at least one mole of water is co-produced and furthermore if water is present in the feed (i.e. crude methanol is charged) or steam is used as a diluent in order to lower the partial pressure of methanol in the reaction zone there can be large amounts of additional steam in the effluent from the MTO reaction zone. The amount of steam in a typical MTO reactor effluent stream is about 40 to 80% of the volume of the effluent stream depending somewhat on the exit temperature and pressure of this effluent stream as well as the degree of conversion and the extent of the use of a water diluent and water contamination of the feed. The operation of a successful MTO process is thus forced to deal with the presence of large amounts of readily condensable material in the effluent gas stream from the MTO conversion zone. This situation necessitates the use of a rather large quench zone on the effluent stream along with associated heat exchange procedures as is explained in my patent U.S. Pat. No. 6,403,854 in order to eliminate substantially all of this water contaminant from this effluent stream to produce a hydrocarbon-rich portion of this effluent stream for further downstream processing. As is explained in some detail in the discussion of FIG. 4 of my '854 patent, a preferred MTO flow scheme employs a product compression zone on this hydrocarbon-rich portion of the effluent gas stream that is recovered from the overhead of the quench zone. Due to the dramatic volume shrinkage that occurs across the quench zone when the water by-product of the MTO reaction is cooled and condensed and due somewhat to the sharp drop in temperature that occurs when the effluent gas stream traverses the quench zone and the associated heat exchangers, the product compression zone operates on an input gas stream which is only about 20 to 60 vol-% of the effluent gas volume originally withdrawn from the MTO conversion zone. Since the product compression zone is sized on the basis of the volume of gas stream that it is expected to handle during on-stream operation, this shrinkage phenomenon that is inherent in the operation of an MTO process is very advantageous when specifying this size of the mechanical compressing means that are used in the compression zone in order to achieve the desired downstream processing pressure. Since the economics of acceptable commercial practice for compression of the hydrocarbon portion of the effluent stream from a commercial scale fluidized MTO unit require the use of one or more variable speed centrifugal compressors that have a limited capacity to handle volumes of gas beyond their design capacity, the opportunity to use the product compression zone as the chief motive force for driving a start-up gas through the fluidized MTO conversion zone appears at first blush to be limited. In other words, it is clear from the discussion above that the process design engineer who attempts to design a start-up protocol for an MTO process using a fluidized conversion zone and to rely on the product compression zone for start-up gas circulation faces a monumental task.
For various reasons well-articulated in UOP's patents U.S. Pat. No. 6,403,854 and U.S. Pat. No. 6,166,282 (all of the teachings of which are hereby specifically incorporated by reference), the consensus of the art relating to the design of an MTO process points to the use of a fluidized reaction zone as the preferred commercial solution to the problem of efficiently and effectively using a SAPO type catalyst system in this type of service. Given this fluidization constraint on the type of reaction system, there is an additional design parameter that must be considered in formulating a start-up protocol for an MTO process using a fluidized MTO conversion zone. Not unexpectedly this design parameter is associated with the catalyst separation technology that is required for proper operation of a fluidized reaction zone. Standard industry practice for fluidized catalyst separation from product gas is shown in the sole drawing of the '854 patent (attached hereto as FIG. 2) and involves the use of one or more cyclones in a disengagement zone typically located above the reaction zone and frequently coupled with a riser termination device that gives an initial gas/solid separation. When an expensive catalyst system such as a SAPO-34 based catalyst system is used (i.e. the SAPO-34 catalyst system is expected to be at least an order of magnitude higher in cost then standard high performance FCC catalyst), it is essential to the economics of the process that catalyst losses be minimized especially during sharp transitions associated with start-up and shutdown of the MTO conversion zone. It is extremely important in other words that the cyclones in the fluidized reaction zone function properly during start-up in order to avoid significant catalyst losses due to excessive attrition induced by start-up turbulence and catalyst blow out caused by operating cyclones at less then there minimum specified superficial gas velocity. Established cyclone design standards require a superficial linear velocity in the inlet throat of the cyclone of about 10.7 to 16.8 m/sec (35 to 50 ft/sec) for proper operation of the cyclone and this requirement is an additional design constraint on the start-up protocol for MTO process using a fluidized MTO conversion zone.
The problem addressed by the present invention is then to design a start-up method for a catalytic MTO process that uses a fluidized MTO conversion zone containing one or more cyclones to separate reaction products from catalyst where the method meets or exceeds the required cyclone minimum superficial linear velocity for initiating catalyst circulation and uses the undersized (i.e. for start-up purposes) motor-driven product compression zone to drive the start-up gas stream through the fluidized MTO conversion zone without employing a large and expensive motor-driven dedicated start-up compressor for start-up gas circulation.